Ink jet printing systems mark a printable medium by propelling droplets of ink onto the medium. Although a number of variations exist in the types of ink jet printing systems that are in use, the two main types of ink jet printing systems are Drop-On-Demand (DOD) printing systems and continuous ejection printing systems. DOD printing systems utilize heating elements or piezoelectric elements within ink ejection nozzles to propel ink on demand onto the medium. Continuous ejection printing systems utilize a continuous stream of ink and electrostatic fields to control the placement of the ink onto the medium.
One potential problem associated with DOD printing systems is that there are periods of time when the ink is not being ejected from the nozzles. When ink is not being ejected, there is little to no flow of ink through the printing system. In this case, ink that is held in various locations within the printing system (e.g., ink in the feed lines and/or reservoirs, and/or print nozzles) can undergo changes in the characteristics of the ink. For example, if the ink is used for Magnetic Ink Character Recognition (MICR) printing, then metal particles used in such inks can settle or stratify within the printing system, which is undesirable. Such settling or stratification can change the ratio of metal particles to liquid in the ink, which can cause printing problems or print quality problems. Further, ink that is held in the nozzles during periods of print inactivity can change viscosity or harden, both of which can clog nozzles or reduce the quality of the printed output of the printing system.
DOD printing systems often attempt to mitigate these issues by ejecting and discarding ink during periods of print inactivity. However, this type of process wastes ink and can increase the operating costs of the printing system.